1. Field of the Invention
The present invention concerns metallic pieces and especially gas turbine blades. In particular, it focuses on repair of the parts of blades which are coated with a layer of an anti-wear material for resisting constraints due to major friction during operation of the motor. The repair process applies to the mobile rotor blades or fixed stator vanes.
2. Discussion of the Background
A mobile turbine blade comprises a root portion by which it is fixed to the disc of the rotor, a platform delimiting the gas vein and an aerofoil portion. According to the motor and the turbine stage, the aerofoil portion terminates, at its end remote from the root, in a transversal element, known as a shroud, which delimits the external surface of the gas vein. The shroud comprises a leading edge and a trailing edge oriented transversally relative to the direction of flow of the gas. The two edges are connected by two lateral edges with which the shroud makes contact with the shrouds of the two adjacent blades on the rotor. These lateral edges have a so-called Z profile: two axial portions are interconnected by a substantially transversal portion.
With the aim of damping the vibrations to which they are subjected during operation, the blades are mounted on the disc with a torsion constraint about their main axis. The shrouds are designed such that the blades are thus placed under torsion constraint when supported with their adjacent blades along said portions transversal surface. Due to the vibrations and strain transmitted between the two surfaces in contact, friction under tension results which is a major factor of wear.
This problem of wear is also encountered on the contact surfaces of the internal platforms of the blades or the distributor sectors. The lateral edges of the blades or the adjacent sectors are in the form of Z likewise with a transverse portion in particular. Due to vibrations these parts are the seat of wear.
The material constituting the blade is generally low resistant to these phenomena of wear in the operating conditions of the motor. In order to prolong the shelf life of the piece, its sensitive parts mentioned hereinabove are protected, by applying a material resistant to wear. This can be small plates of a specific alloy exhibiting increased hardness which is brazed onto the metallic substrate, as is described in the U.S. Pat. No. 6,164,916 and which form the support surfaces with the adjacent pieces. According to another known technique, to manufacture new pieces progressive loading of the surface to be protected is commenced possibly after the material of the substrate has been removed locally by grinding, to a determined depth, by means of a rod of an anti-wear material which is fused at the same time as the upper layer of the metallic substrate. The necessary heat is contributed by means of an appropriate source. This can be produced by an electric arc sheath of inert gas or else by a laser beam.